Impacts of environmental factors on Bowen ratio in typical farmland ecosystem in the Loess Plateau

The Bowen ratio can comprehensively reflect physical characteristics of land surface climate state and is one of the key parameters that describe water and heat distribution of the ecosystem. In this paper, the eddy correlation systems installed in Dingxi and Qingyang are used to carry out the observation experiment on energy distribution characteristics in semi-arid and semi-humid farmland ecosystem over the Loess Plateau. Meanwhile, the influence mechanism of eco-environmental factors on the Bowen ratio is studied. This study also reveals the response law of physiological and ecological factors to water and heat exchanges under dry and wet conditions. The results show that sensible heat flux in Dingxi, a semi-arid region, is the main consumption item of available energy. Even in the monsoon season when precipitation is relatively concentrated, its Bowen ratio still fluctuates around 1. For the situation in Qingyang, which is located in the semi-humid area, latent heat flux in summer plays a dominant role in energy distribution, and sensible heat fluxes dominates the energy distribution in the rest of the season (the Bowen ratio is between 1.15—5.85). From the perspective of meteorological factors that affect land surface water and heat exchanges, the Bowen ratio increases with increasing VPD (vapor pressure deficit), and decreases significantly with increasing precipitation and soil moisture. Under arid condition, the Bowen ratio has a higher correlation with VPD in semi-humid areas (R²=0.44) and a higher correlation with VPD in semi-arid areas (R²=0.38) under humid conditions. In addition, the Bowen ratio in the semi-arid region is 1.5 times higher than that in the semi-humid region during the growing season. In semi-humid areas, there is a significant correlation between precipitation and the Bowen ratio under both dry and wet conditions, and the determination coefficient even reaches 0.79 under drought condition. However, in semi-arid areas, the correlation is significant (R²=0.40) only in drought condition. The correlation between soil water content (SWC) and the Bowen ratio is more significant in semi-humid areas, and the Bowen ratio increases with decreasing SWC under drought condition. From the perspective of ecological factors that affect Bowen ratio in the growing season, the relationship between the Priestley-Taylor coefficient (\alpha ) and the Bowen ratio satisfies the power function law. There is a high correlation in the farmland ecosystem over the Loess Plateau, and the determination coefficients for semi-humid and semi-arid areas are 0.62 and 0.72 respectively. The Bowen ratio decreases with increasing NDVI (Normalized Difference Vegetation Index), and this phenomenon is more obvious in the semi-humid zone (R²=0.40). However, the response of the Bowen ratio to NDVI in semi-arid area is faster than that in semi-humid area. There is a negative exponential relationship between the canopy stomatal conductance (G_s) and the Bowen ratio. The Bowen ratio has a declining trend with increasing G_s in semi-arid areas than in semi-humid areas. This study reveals the land surface characteristics in typical farmland ecosystem over the Loess Plateau and provides an important reference for improving the parameterization of land surface process.